1. Field of the Invention
The invention relates to a method for manufacturing a telescopic shaft.
2. Description of Related Art
Each of telescopic shafts described in Japanese Patent Application Publication No. 2013-213589 and Japanese Patent Application Publication No. 2012-102868 has a male shaft (an inner shaft), a female shaft (an outer shaft), and a coating portion. The male shaft has a non-circular outer peripheral shape. The female shaft has a non-circular inner peripheral shape. Forming the coating portion on an outer periphery of the male shaft reduces sliding resistance between the outer periphery of the male shaft and an inner periphery of the female shaft. When the telescopic shaft is manufactured, the inner periphery of the female shaft is fitted around the outer periphery of the male shaft with interference therebetween, and the male shaft is then reciprocally slid relative to the female shaft fixed by a processing tool a plurality of number of times in an axial direction. The coating portion is plastically deformed by frictional heat generated by sliding, whereby the interference is reduced. Therefore, the sliding resistance between the female shaft and male shaft can be set to a predetermined value.
In JP 2013-213589 A and JP 2012-102868 A, when the male shaft starts sliding relative to the female shaft, the plastic deformation of the coating portion has not occurred and the coating portion is thick. Therefore, the interference between the inner periphery of the female shaft and the outer periphery of the male shaft is large. Under the influence of such interference and inertia of a moving part in a facility for sliding the male shaft, when the male shaft starts sliding relative to the female shaft, a sliding load of the male shaft is large.
In order to withstand the sliding load, various parts of each of the male and female shafts are required to ensure the strength larger than a predetermined strength. For securing the strength, there is a limit to reduction in cost and weight of the telescopic shaft. In order to reduce the cost and the weight of the telescopic shaft, the sliding load is required to be reduced. If the interference is reduced, the clearance between the female shaft and the male shaft becomes the positive clearance in the worst case, whereby the plastic deformation and welding deformation of the coating portion by the frictional heat cannot be achieved. Alternatively, as a means for reducing the sliding load, it is conceived that a weight of the moving part is reduced to reduce the influence of the inertia of the moving part in the facility. However, it may be difficult to modify the facility due to durability of the facility and the like.
As the means for reducing the sliding load, reducing the interference or reducing the weight of the moving part in the facility is difficult.